When you navigate to Manage->Computers do you have the blue Silverlight UI? If so, select All Computers and then next to the search field in the second column, select the little chevron to show the option "Add Search Criteria." Click on it, and the select "Add Custom Criteria...." When the windows comes up, under DataClasses, look for AeX AC TCPIP. Select the box next to it. On the ride hand side, under Columns, you should now see lots of different column names under AeX AC TCPIP. Select the column "MAC Address." Hit OK and the window should disappear. Now, select Add Search Criteria again and choose "MAC Address" from the list of options. In the empty white field, input the MAC Address for which you are searching.

*Note, if you navigate away from this page, the MAC Address custom search criteria will be removed. Hit the blue "Save" icon so that you can easily search for this in the future.